The present invention relates generally to color television picture tubes used in color reproduction and, more particularly, to color television picture tubes affording increased brightness.
In color television picture tubes of the type with which the present invention is concerned, it is common to provide an evacuated envelope made of glass having an enlarged end carrying a faceplate, and at its opposite end a neck portion containing an electron gun for projecting a beam of electrons toward an image screen on the rear surface of the faceplate. The energy of the electrons is converted into light by the image screen which is comprised of a suitable phosphor layer. The color television picture tube is preferably provided with an image screen which is differentiated from point to point in that adjacent elements of different phosphor material produce light of different colors. A color-selection electrode, usually a multi-apertured mask and conventionally made of a very thin metal sheet which is opaque to the passage of electrons, is disposed between the electron gun and the image screen and is adjacent to the screen. The mask is provided with a large number of small, closely spaced apertures geometrically related to the different phosphor elements on the screen in a predetermined manner. The relation of the apertures to the phosphor elements and the electron beam sources is such that, by appropriate placement of the electron beam apparent sources, different phosphor elements, producing predetermined color emission, can be selectively energized to produce a visible picture corresponding to the original scene.
In some color tubes, the apertures of the mask are of such size that the electron beam impinges only a portion of the respective phosphor elements. The difference between the actual size of the phosphor element and the area impinged by the electron beam constitutes a misregistration tolerance condition or a guard ring which provides a safety factor for preventing color contamination owing to various mechanical, thermal and electrical errors. This tolerance condition is termed a positive misregistration tolerance condition.
A color television picture tube, disclosed in Patent No. 3,146,368, comprises within an evacuated envelope: a multi-color image screen including a plurality of interspersed groups of similarly shaped phosphor elements, each of the phosphor elements being spaced from all adjacent such elements by intermediate light absorbing material; electron gun means for projecting a corresponding plurality of electron beam components towards the image screen; and means, including a color-selection electrode provided with a plurality of apertures individually larger or wider than the phosphor elements and disposed between the screen and the electron gun means, causing a beam landing area larger (or wider) than the size of the phosphor element, and thus establishing a negative misregistration tolerance condition.
The referent copending application discloses and claims a color television picture tube having an image screen including a pattern of red-light-emitting, blue-light-emitting and green-light-emitting phosphor elements. Electron gun means generates first, second and third angularly spaced electron beams for exciting the phosphor elements. A color-selection electrode is disposed adjacent the screen and has a pattern of apertures for causing the first, second and third electron beams to impinge exclusively on the red-light-emitting, blue-light-emitting and green-light-emitting phosphor elements, respectively. The apertures in the color-selection electrode and the phosphor elements are dimensioned relative to each other such that beam landing areas formed by the mask apertures and the respectively associated phosphor elements have a relative dimensional difference which varies between the central portion and the peripheral portions of the image screen from a condition in which the beam landing areas have a greater dimension than the associated phosphor elements (negative tolerance condition) to a condition in which the beam landing areas have a dimension smaller than the associated phosphor elements (positive tolerence condition). The screen has light absorptive material between the phosphor elements in at least those portions of the screen wherein the beam landing area dimension is larger than the associated phosphor element dimension.
In a conventional color television picture tube having an electron gun assembly the first gun to bloom marks the limit of the tube drive since current-limited circuitry is not involved. The green-light-emissive phosphor element takes the most beam current and therefore the electron gun which energizes the green-light-emissive phosphor element is the first to bloom. This limit on brightness occurs in color television picture tubes irrespective of whether they have positive tolerance conditions or negative tolerance conditions.